When a picture has noise it means that it has these little grains on the picture and it make the quality of the picture bad. 
This occurs when you take a picture with a high ISO, usually 6400+ and it varies on some cameras. It it most common in night/dark photography because usually you need more light at night.
This is a comparison between a clear and noisy image. To avoid a noisy image you want to try to shoot at a low ISO and use a lower shutter speed until the image is clear.
Thanks for this tip, I´ll definitely use this!!